1. Field of the Invention
The present invention relates to semiconductor device, and more particularly, to a method for fabricating a compound semiconductor device.
2. Discussion of the Related Art
In general, a recess etching technique is widely applied to the fabrication of a GaAs FET, one of the compound semiconductor devices. The recess etching is applied to the etching of an active layer to form a gate electrode on the active layer in formation of a gate pattern of the compound semiconductor device. A thickness of the active layer is dependent on a ratio of etching of the recess, which thick, iss is closely related to a threshold voltage of the compound semiconductor device. The relation between the thickness of the active layer and the threshold voltage can be expressed with an equation shown below. ##EQU1##
Where, D is the active layer thickness, Nd is a doping concentration of the active layer, Vbi is a built-in voltage of a Shottky contact, .epsilon. is a dielectric constant of GaAs, and Vth is the threshold voltage. It can be known from the equation that the thicker the active layer, the higher the threshold voltage, and the thinner the active layer, the lower the threshold voltage.
A conventional method for fabricating a compound semiconductor device applying such a recess etching technique thereto will be explained with reference to the attached drawings.
FIGS. 1a.about.1e illustrate sections showing steps of a conventional method for fabricating an MESFET(Metal Semiconductor Field Effect Transistor).
Referring to FIG. 1a, an undoped GaAs buffer layer 2, an n-GaAs active layer 3 and an n.sup.+ -GaAs cap layer 4 are formed on a semi-insulating GaAs substrate 1 in succession, and a predetermined portion of each of the cap layer 4, the active layer 3 and the buffer layer 2 are subjected to mesa etching for isolation between devices. Referring to FIG. 1b, the source and drain electrodes 5 and 6 are formed in predetermined regions on the cap layer 4, and, as shown in FIG. 1c, a photoresist 7 is formed on the entire surface of the substrate 1 including the source and drain electrodes 5 and 6 and subjected to patterning to open a portion of the cap layer 4 between the source and drain electrodes 5 and 6. The opened cap layer 4 and the underlying active layer 3 are wet etched to a predetermined depth to form a recess. Referring to FIG. 1d, a gate metal 8 is formed on the entire surface of the opened active layer 3 and the photoresist 7, and, as shown in FIG. 1e, the photoresist 7 and the overlying gate metal 8 are lifted-off to form a gate electrode 8a on the active layer 3, thereby an MESFET device is fabricated.
However, the aforementioned conventional method for fabricating a compound semiconductor device has the following problem.
The individual formation of devices each having threshold voltages different from one another in providing a circuit requiring a variety of threshold voltages causes the fabrication cost high.